The new JPEG 2000 decoding standard (ITU-T Rec.T.800/ISO/IEC 154441:2000 JPEG 2000 Image Coding System) provides a new coding scheme and codestream definition for images. Although the JPEG 2000 standard is a decoding standard, the JPEG 2000 specifies encoding and decoding by defining what a decoder must do. Under the JPEG 2000 Standard, each image is divided into one or more rectangular tiles. If there is more than one tile, the tiling of the image creates tile-components that can be extracted or decoded independently of each other. Tile-components comprise all of the samples of a given component in a tile. An image may have multiple components. Each of such components comprises a two-dimensional array of samples. For example, a color image might have red, green and blue components.
After tiling of an image, the tile-components may be decomposed into different decomposition levels using a wavelet transformation. These decomposition levels contain a number of subbands populated with coefficients that describe the horizontal and vertical spatial frequency characteristics of the original tile-components. The coefficients provide frequency information about a local area, rather than across the entire image. That is, a small number of coefficients completely describe a single sample. A decomposition level is related to the next decomposition level by a spatial factor of two, such that each successive decomposition level of the subbands has approximately half the horizontal resolution and half the vertical resolution of the previous decomposition level.
Although there are as many coefficients as there are samples, the information content tends to be concentrated in just a few coefficients. Through quantization, the information content of a large number of coefficients is further reduced. Additional processing by an entropy coder reduces the number of bits required to represent these quantized coefficients, sometimes significantly compared to the original image.
The individual subbands of a tile-component are further divided into code-blocks. These code blocks can be grouped into partitions. These rectangular arrays of coefficients can be extracted independently. The individual bit-planes of the coefficients in a code-block are entropy coded with three coding passes. Each of these coding passes collects contextual information about the bit-plane compressed image data.
The bit stream compressed image data created from these coding passes is grouped in layers. Layers are arbitrary groupings of successive coding passes from code-blocks. Although there is great flexibility in layering, the premise is that each successive layer contributes to a higher quality image. Subband coefficients at each resolution level are partitioned into rectangular areas called precincts.
Packets are a fundamental unit of the compressed codestream. A packet contains compressed image data from one layer of a precinct of one resolution level of one tile-component. These packets are placed in a defined order in the codestream.
The codestream relating to a tile, organized in packets, are arranged in one, or more, tile-parts. A tile-part header, comprised of a series of markers and marker segments, or tags, contains information about the various mechanisms and coding styles that are needed to locate, extract, decode, and reconstruct every tile-component. At the beginning of the entire codestream is a main header, comprised of markers and marker segments, that offers similar information as well as information about the original image.
The codestream is optionally wrapped in a file format that allows applications to interpret the meaning of, and other information about, the image. The file format may contain data besides the codestream.
The decoding of a JPEG 2000 codestream is performed by reversing the order of the encoding steps. FIG. 1 is a block diagram of the JPEG 2000 standard decoding scheme that operates on a compressed image data codestream. Referring to FIG. 1, a bitstream initially is received by data ordering block 101 that regroups layers and subband coefficients. Arithmetic coder 102 uses contextual information collected during encoding about the bit-plane compressed image data, and its internal state, to decode a compressed bit stream.
After arithmetic decoding, the coefficients undergo bit modeling in coefficient bit modeling block 103. Next, the codestream is quantized by quantization block 104, which may be quantizing based on a region of interest (ROI) as indicated by ROI block 105. After quantization, an inverse transform is applied to the remaining coefficients via transform block 106, followed by DC and optional component transform block 107. This results in generation of a reconstructed image.
The JPEG2000 standard leaves many choices to implementers.